A voltage booster device includes a voltage booster circuit, which outputs a booster output voltage. The booster output voltage outputted by the booster circuit, is used in an inverter circuit for driving an AC motor, for example. By using the booster circuit, influence of resistance of a wire between an inverter and a motor and influence of variation in a battery voltage are reduced. In case of using the booster circuit in the electric power steering system, stability of steering operation is enhanced.
Patent document 1 (JP 4483322) and patent document 2 (JP 2006-62515A, US 2006/0044852A1) disclose exemplary voltage booster devices, which are used in an electric power steering system. A chopping-type voltage booster circuit used in the exemplary voltage booster device performs a voltage boosting operation by turning on and off switching elements at high speeds. The voltage booster circuit controls a booster output voltage by regulating a voltage boosting duty, which is a ratio of the on-period of the voltage boosting switching element relative to a one-cycle period of the switching operation. The voltage booster device according to patent document 1 reduces the output voltage of the booster circuit to suppress heat generation of the booster circuit when the booster output voltage exceeds an upper limit voltage. The booster device according to patent document 2 corrects a target voltage to suppress overshooting of the output voltage.
According to the booster devices disclosed in patent documents 1 and 2, the booster output voltage falls and the target voltage cannot be provided when high output power exceeding a power corresponding to a saturation duty, which is an upper limit value of a boosting duty. In case that this booster circuit is used in an electric power steering system, assist torque required for rapid steering operation time cannot be provided and steering operation feeling is worsened. In case of an electric power steering system, which is used for a large-sized vehicle, it is required to be able to output high power.
One exemplary conventional motor drive apparatus is shown in FIG. 6. As shown in FIG. 7, a booster circuit 70 of a motor drive apparatus 7 includes a booster coil 71, a boosting switching element 72, a reducing switching element 73 and an output capacitor 74. The booster circuit 70 performs voltage boosting operation by turning on and off the switching elements 72, 73 at high speeds. Thus, the booster circuit 70 is formed as a chopping-type booster circuit.
The booster coil 71 induces voltage in response to charge and discharge of energy. The boosting switching element 72 and the reducing switching element 73 are formed of, for example, MOS field-effect transistors, and turned on and off by electric signals supplied from a CPU 21. The boosting switching element 72 is connected between an output terminal of the booster coil 71 and the ground. The reducing switching element 73 is connected between the output terminal of the booster coil 71 and the motor driver circuit 25. The output capacitor 74 is connected between an output terminal of the booster coil 73 and the ground to smooth a booster output voltage Vout. With the above-described configuration, the booster circuit 70 boosts the battery voltage Vin of the battery 22 and outputs the booster output voltage Vout to the motor drive circuit 25.
In operation, the boosting switching element 72 and the reducing switching element 73 in the booster circuit 70 are controlled to turn on and off alternately in accordance with on/off control signals from the CPU 21. That is, one of the switching elements 72, 73 is turned on when the other of the switching elements 72, 73 is turned off. This is for the purpose of protecting switching elements 72, 73 from being broken due to excessive current, which flows when both the boosting switching element 72 and the reducing switching element 73 are turned on at the same time. Since the switching elements 72, 73 are both turned on for a short time at the same time because of short time delay in on/off switching-over time, a dead time is provided so that both switching elements 72, 73 are turned off. No dead time is assumed here.
When the boosting switching element 72 is switched over from the on-state to the off-state, current flows from the battery 22 to the booster coil 71. The magnetic field generated by the booster coil 71 changes in correspondence to changes in the current and induces voltage, and energy is charge in the booster coil 71. When the boosting switching element 72 is switched back to the off-state and the reducing switching element 73 is switched over to the on-state, the induced voltage of the booster coil 71 is superimposed on the battery voltage Vin. The booster coil 71 thus charges the output capacitor 74 while discharging its charged energy. The booster output voltage Vout is thus raised by the repetition of the switching operation.
The CPU 21 receives the booster output voltage Vout, which is fed back, and controls the duty of the switching operation so that the booster output voltage Vout attains a target output voltage Va of the booster circuit 70. The duty is a ratio of the on-time of the switching element in a one-cycle time of the switching operation and expressed in units of %. The duty of the boosting switching element 72 is referred to as a boosting duty D1, and the ratio of on-time of the reducing switching element 73 is referred to as a reducing duty D2. The CPU 21 increases the boosting duty D1 to promote a voltage boosting operation when the booster output voltage Vout is lower than the target output voltage Va. The CPU 21 reduces the boosting duty D1 to suppress the voltage boosting operation when the booster output voltage Vout approaches the target output voltage Va.
The relation among the boosting duty D1, the reducing duty D2, and the boosting ratio α, which indicates a ratio of the booster output voltage Vout relative to the battery voltage Vin and are expressed by equations (1-1) and (1-2).Vout=Vin×α  (1-1)α=(D1+D2)/(100−D1)  (1-2)
The following equation (1-3) holds, if dead-time is ignored. The equation (1-2) is expressed as equation (1-4).D1+D2=100  (1-3)α=(D1+D2)/D2  (1-4)
For example, α=2 holds if D1=D2=50%.
In this case, the booster output voltage Vout becomes two times as large as the battery voltage Vin.
The conventional booster circuit 70 thus exhibits an output voltage characteristic and a duty characteristic as shown in FIG. 7. In FIG. 7, the abscissa axis indicates an inverter output, that is output (unit: W) supplied to the motor drive circuit 25 and the ordinate axis indicates voltages (unit: V) and duties (unit: %). The voltages include the battery voltage Vin, the booster output voltage Vout and the target output voltage Va. The duties include the boosting duty D1, the reducing duty D2 and a saturation duty Dmax.
It is assumed in the conventional booster circuit 70 that the inverter output is Q1, the boosting duty D1 is about 70% and the reducing duty D2 is about 30%. With this assumption, the boosting ratio α is about 3.3 from the equation (1-4). Therefore, the booster output voltage Vout becomes about 33V relative to the battery voltage Vin of about 10V. Assuming that the target output voltage Va is fixed at, about 33V, the CPU 21 increases the boosting duty D1 in accordance with an increase in the required inverter output so that the booster output voltage Vout attains the target output voltage Va. As the boosting duty D1 increases, the reducing duty D2 decreases. However, the boosting duty D1 is limited at its upper limit, which is the saturation duty Dmax provided to protect the boosting switching element 72 from breaking down. In this example, the saturation duty Dmax is set to about 80%. The boosting duty D1 therefore becomes a fixed value over a limit output Qc of the inverter. As a result, the booster output voltage Vout falls below the target output voltage Va.
If the target output voltage Va is not supplied to the motor drive circuit 25, the motor drive apparatus 2 cannot drive the motor 80 satisfactorily. If the required inverter output becomes greater than the limit output Qc when the greatest assist torque is required for rapid steering operation or the like in the electric power steering apparatus 1, the required assist torque cannot be produced and steering operation feeling will be worsened.